1. Field of the Invention
The present invention relates to a semiconductor electronic device made of a nitride-based compound semiconductor.
2. Description of the Related Art
The electronic devices such as the field effect transistor or the like using a nitride based compound semiconductor expressed by a formula AlxInyGa1-x-yAsuPvN1-u-v (where 0≦x≦1, 0≦y≦1, x+y≦1, 0≦u≦1, 0≦v≦1, u+v<1), such as a GAN-based compound semiconductor, has attracted a significant attention as a solid state device capable of operating in a high temperature environment close to 400° C. Unlike Si or GaAs, it is difficult to manufacture a large diameter single crystal substrate of GaN-based compound semiconductor. Therefore, the electronic device using a GaN-based compound semiconductor is fabricated on a substrate formed of silicon carbide (SiC), sapphire, ZnO or Si. Especially, Si substrate is very useful because a large diameter wafer is available at a low price.
However, there are large differences in lattice constant and thermal expansion coefficient between Si and GaN. Therefore, when a GaN layer is grown directly on a Si substrate, a large tensile strain is created in the GaN layer, which is a cause of a concave warp of the epitaxial wafer and/or a degradation of a crystal quality. Further, when the immanent strain is too large, cracks are generated in the GaN layer. Therefore, a buffer layer is normally provided between the Si substrate and the GaN layer as a strain relaxing layer. A lamination structure constituted by GaN layers and AlN layers is effective as the buffer layer (see Patent Reference 1 and 2).
A manufacturing method of a GaN-based field effect transistor described in the Patent Reference 1 is as follows. First, an AlN layer is formed on a single crystal Si substrate of 4 inches (101.6 mm) in diameter using an epitaxial crystal growth method such as MOCVD method at a substrate temperature of about 1000 to 1100° C. Then, a buffer layer is formed by growing composite layers in which a GaN layer and an AlN layer are laminated at the same temperature. Thereafter, an electron drift layer, an electron supplying layer and a contact layer are sequentially grown on the buffer layer to form a semiconductor active layer, and a source electrode, a drain electrode, and a gate electrode are formed. Then, the wafer is separated into each device. In this way, by forming the buffer layer constituted by composite layers of GaN layer and AlN layer, it is possible to epitaxially grow a GaN layer with no crack included and of superior crystal quality on the Si substrate. Further, a warp of the epitaxial wafer can be reduced. Note that the buffer layer is not limited to the composite layers of GaN layer and AlN layer, but may be composite layers of AlGaN layers having different compositions each other and appropriate amount of strain therebetween.    Patent Reference 1 Japanese patent publication No. 2003-59948    Patent Reference 2 Japanese patent publication No. 2007-88426
In order to realize an electric power source device employing an electronic device that includes epitaxial layers of a GaN-based compound semiconductor, it is important to increase a breakdown voltage of the electronic device. A Si substrate has relatively a low resistance as compared with a sapphire substrate, for example. Accordingly, in order to increase a breakdown voltage of the electronic devices using a Si substrate, it is necessary to increase a total layer thickness of the epitaxial layers formed on the Si substrate. However, when increasing the total layer thickness of the epitaxial layers, an amount of strain included also increases. Therefore, in order to prevent an adverse effect of the strain, it is necessary to increase the number of composite layers of the buffer layer according to the increase in the total layer thickness of the epitaxial layer.
However, the increase in the number of composite layers will cause convex warp of the epitaxial wafer to a large degree. Therefore, it is difficult to increase a breakdown voltage while restraining the warp of the epitaxial wafer.